Solve 2 sin^2 x + 3 sin x = -1 Step by Step

Step by step

The question

Question: solve 2 sin² x + 3 sin x = −1.

Find every angle from 0° up to 360°.

2 sin² x + 3 sin x = −1

We want the angles from 0° to 360°: one full turn.

0° ≤ x < 360°

1 Get 0 on one side

Everything goes on the left, so the right side is 0.

We take away −1 on both sides, so they stay equal.

2 sin² x + 3 sin x + 1 = 0

2 Name it

Look: sin² x and sin x.

Like x² and x in a quadratic.

We call sin x by one letter: u = sin x.

u = sin x

Then the equation is 2u² + 3u + 1 = 0: a quadratic.

We solve it the usual way.

2u² + 3u + 1 = 0

3 Find two numbers

There is a number in front of u², but it can still factor.

The number in front of u² is 2.

So we multiply 2 by 1: that is 2.

We need two numbers that multiply to 2 and add to 3.

2 × 1 = 2

? × ? = 2, ? + ? = 3

They multiply to a plus number, so the two numbers have the same sign.

They add to a plus number, so both are plus.

× is + → same signs: both +

We list the pairs that make 2, smallest first.

We add each pair.

We need 3.

1 and 2 give 3.

Yes!

1 × 2: 1 + 2 = 3 ✓

4 Split the middle

We write 3u as u + 2u.

It is the same amount.

2u² + u + 2u + 1 = 0

5 Group in pairs

We put the first two together, and the last two together.

(2u² + u) + (2u + 1) = 0

From the first pair we take out u.

From the second pair we take out 1.

Both leave the same bracket.

(2u² + u) = u(2u + 1)

(2u + 1) = 1(2u + 1)

Now both parts have 2u + 1 in them.

u(2u + 1) + 1(2u + 1) = 0

6 Same bracket out

2u + 1 is in both parts, so we take it out once.

What is left, u + 1, goes in the other bracket.

ab + cb = b(a + c)

So we get (2u + 1)(u + 1).

(2u + 1)(u + 1) = 0

7 One bracket is 0

Two numbers multiply to 0.

That only happens if one of them is 0.

So one bracket must be 0.

A × B = 0 → A = 0 or B = 0

We make each bracket 0.

Two small equations.

2u + 1 = 0 or u + 1 = 0

In 2u + 1 = 0, we take away 1 on both sides.

In u + 1 = 0, we take away 1 on both sides.

2u + 1 − 1 = 0 − 1 or u + 1 − 1 = 0 − 1

Now we have 2u = −1 or u = −1.

2u = −1 or u = −1

To remove times 2, we divide both sides by 2.

So u = −1/2 or u = −1.

u = −12 or u = −1

8 Put sin x back

But u is sin x.

So sin x = −1 or sin x = −1/2.

sin x = −1 or −12

Careful!

These are not the answers yet!

They are values of sin x.

We still need the angles.

Careful: not x = −1 or −12 ✗

9 Find every angle

From the unit circle: sin 30° = 1/2.

So the reference angle is 30°.

The minus only changes which quadrants.

sin 30° = 12

Sin is the up value.

The points with that height are on a flat line.

The up value is −1 at one point only.

x = 270°.

sin x = −1: x = 270°

Sin is negative below the x-axis: quadrants III and IV.

180° + 30° and 360° − 30°: x = 210° or 330°.

sin x = −12: x = 210° or 330°

On the circle: 3 points.

All together, in order: x = 210°, 270°, 330°.

x = 210°, 270°, 330°

10 Check

Let's check every answer in 2 sin² x + 3 sin x + 1 = 0.

sin x is −1/2 at 210°.

Each one must give 0.

x = 210°: 2(−12)² + 3(−12) + 1 = 12 − 32 + 1 = 0

x = 270°: 2(−1)² + 3(−1) + 1 = 2 − 3 + 1 = 0

x = 330°: 2(−12)² + 3(−12) + 1 = 12 − 32 + 1 = 0

Every answer works.

So x = 210°, 270°, 330°.

Let's look back.

First: get 0 on one side.

Then: name it.

Then: find two numbers.

Then: split the middle.

Then: group in pairs.

Then: same bracket out.

Then: one bracket is 0.

Then: put sin x back.

Then: find every angle.

Last: check.

Answer